1. Field of the Invention
The present invention relates to a vehicle diagnostic system.
2. Description of the Related Art
Modern vehicles incorporate complex electronic and electromagnetic active components such as engine management systems, fuel delivery control systems, and brake control system. It is desirable that these active components intercommunicate and as a result it is now becoming conventional to equip vehicles with a serial data communication bus linking active vehicle components and in particular components known as Electronic Control Units (ECU's). There are several widely used serial communication protocols which are accepted as industry standards, one such high speed and high performance protocol being known as CAN (that is Controller Area Network). The term CAN bus is used herein to mean any bus linking active components of a vehicle and conveying data representative of the performance of those components.
It is conventional practice to communicate between diagnostic equipment and a vehicle over its diagnostic bus. However, a CAN bus is not primarily intended for diagnostics and is not designed to allow for the connection of additional cable lengths to the vehicle's wiring system. The addition of a length of cable to a vehicle's CAN bus may cause data corruption.
The active components transmit to and receive data from the CAN bus to which they are connected at a high data rate, for example 1 Mbps. Such a high data rate ensures that the large volume of data can be handled, but also means that propagation delays on the bus must be minimized. Propagation delays of more than 20 ns are not acceptable on a conventional CAN bus. Accordingly, each active element must be carefully designed to present no more than a predetermined electrical load to the bus.
In order to gain access to data on a CAN bus for diagnostic purposes, it is conventional practice to connect a gateway terminal to the bus. The gateway terminal is permanently connected to the bus and in effect represents an additional active component. The gateway terminal monitors data on the CAN bus, and records fault conditions or other diagnostic related parameters. This diagnostic data may later be downloaded to appropriate diagnostic equipment. The data available at the output of the gateway terminal is limited in scope as compared with that on the bus, the nature of the data being determined largely by regulatory demands, e.g. regulations imposed by particular political bodies. Typically a gateway terminal is designed to meet an agreed standard such as ISO9141. Unfortunately, although regulations may change over the life of a vehicle, it is not possible to readily modify a gateway terminal to reflect such changes. Furthermore, problems can arise in a vehicle which cannot be readily diagnosed using the signals output by a conventional gateway terminal, especially where the data rate on the vehicle side of the gateway is much higher than that on the output side of the gateway.
A CAN bus is generally in the form of a twisted pair cable linking the active components. It is sometimes desirable to connect diagnostic equipment to the cable to gain direct access to the data on the bus rather than relying upon a gateway terminal. Unfortunately, diagnostic equipment is generally located in cabinets and cannot readily be positioned close to a vehicle under test. It has been discovered by the applicants that it is not possible to connect a cable between a CAN bus and a remote diagnostic station as signal reflections arise which result in corruption of data on the bus. In a typical CAN system the maximum acceptable length of a stub connection to the bus is limited to 30 centimeters. This makes a direct connection between a CAN bus and many types of diagnostic equipment impractical.